1. Field of the Invention
The present invention relates to a servomotor current control method, a current control program, a recording medium, a servomotor and an injection molding machine. More particularly, the present invention relates to a current control method, a current control program, a recording medium, a servomotor and an injection molding machine to prevent an armature voltage saturation by supplying a d-axis current to a dq-converted armature of the servomotor.
2. Description of Related Art
In order to prevent an armature voltage saturation of a servomotor, a d-axis current Id is supplied to a dq-converted armature where a d-axis direction coincides with a direction of magnetic field flux (for example, see Document 1: JP-9-84400 (Pages 2 to 4, FIGS. 2 and 9)). In the servomotor, a q-axis current Iq is an active current that generates a torque while the d-axis current Id is a reactive current that does not contribute to the generation of the torque. However, by supplying the reactive d-axis current Id, the influence of back electromotive force that occurs in an armature can be reduced, thereby preventing the voltage saturation. Thus, a larger amount of the q-axis current Iq can be supplied. Consequently, the current and torque can be stably controlled.
The above arrangements are shown in FIGS. 2 and 9 of Document 1. FIGS. 2 and 9 show a d-axis voltage Vd and a q-axis voltage Vq which are mutually orthogonal and divided from an armature voltage. A total vector sum of the d-axis voltage Vd and q-axis voltage Vq is equivalent to a total armature voltage applied to the armature. A circle shown in each drawing represents a link voltage that defines an upper limit of the total armature voltage. Accordingly, when the end of the total armature voltage vector falls within the link voltage circle, a desirable armature voltage can be applied to the armature in accordance with the indication of each drawing. Conversely, when the end of the total armature voltage vector goes out of the link voltage circle, the voltage saturation occurs. Thus, the desirable voltage cannot be applied to the armature in accordance with the indication of each drawing, which means that a desirable q-axis current Iq for generating the toque cannot be supplied to the armature.
In the drawings, back electromotive force E, which is a vector of a +q-axis direction, is expressed by a known equation E=ω·Φ for a synchronous servomotor. In this equation, ω denotes a rotation angular speed of the servomotor, and Φ denotes total magnetic flux that is intersected with armature windings. (The following will be described assuming that ω≧0 and Φ≧0, unless otherwise noted.)
Since the back electromotive force E (=ω·Φ) is proportional to ω, the back electromotive force E is increased as the servomotor rotates at higher speed. In FIG. 9, when the back electromotive force vector E is increased during the high-speed rotation, the end of the vector approaches the circumference of the link voltage circle, so that a large amount of the q-axis current Iq cannot be supplied. This is because, when the large amount of the q-axis current Iq is supplied, a drive voltage vector of a +q-axis direction (the same direction as E) expressed as R·Iq in FIG. 9 is elongated, so that the end of the vector sum E+R·Iq goes out of the link voltage circle and therefore the voltage saturation occurs.
However, when the d-axis current Id (≦0) is supplied at this time as shown in FIG. 2, the above-described problem does not occur. This is because, by supplying the d-axis current Id, an offset voltage vector −ω·L·|Id| can be generated in a direction opposite to the back electromotive force vector E (−q-axis direction). Due to this offset voltage vector of the −q-axis direction, the end of the total armature voltage vector, i.e., the vector sum, can stay within the link voltage circle even when the drive voltage vector R·Iq of the +q-axis direction is elongated. Accordingly, when the d-axis current Id is supplied, the large amount of the q-axis current Iq can be continuously supplied even during the high-speed rotation. Thus, a large torque (∝Iq) can be stably and constantly generated.
Further, in order to prevent a voltage saturation using a d-axis current Id, a current control method in which an, amount of the d-axis current Id is controlled based on a voltage command value for an armature winding has been known. According to this method, the voltage saturation can be appropriately prevented without supplying an extra amount of the d-axis current Id (for example, see Document 2: JP-A-2007-151294).
In the servomotor disclosed in Document 1, since the back electromotive force E (=ω·Φ) is proportional to the rotation angular speed ω, the d-axis current Id is defined as a one variable function only depending on the rotation angular speed ω. Then, the voltage saturation is prevented by supplying the d-axis current Id that is increased in a negative direction as the rotation angular speed ω is increased. (Hereinafter, the d-axis current Id is supplied always in the negative direction. Thus, “increase in the negative direction” and “decrease in the negative direction” are simply referred to as “increase” and “decrease” in the following description.)
In the servomotor disclosed in Document 1, the large amount of the d-axis current Id is continuously supplied during the high-speed rotation (ω: large). However, even during the high-speed rotation, a generated torque is small when a load applied to the servomotor is low. Accordingly, the q-axis current Iq (∝ torque) can be reduced. Then, the drive voltage vector R·Iq can be shortened in FIG. 9, so that the end of the total armature voltage vector, i.e., the vector sum, can stay within the link voltage circle. Consequently, the voltage saturation is hardly generated. At this time, the offset voltage vector −ω·L·|Id| that prevents the voltage saturation can be also reduced. Thus, the large amount of the d-axis current Id does not need to be supplied. As described above, it is not necessary that the large amount of the d-axis current Id is continuously supplied even during the high-speed rotation in order to prevent the voltage saturation as in the servomotor disclosed in Document 1. Since an extra amount of the d-axis current Id that is originally not required is continuously supplied, excess heat that need not originally be generated is constantly generated in the servomotor disclosed in Document 1. As a result, various problems may be occurred. For instance, additional compensatory measures may be required in view of maintenance management of the servomotor, or energy efficiency of the servomotor may be deteriorated.
Also, in the servomotor disclosed in Document 1, the d-axis current Id is small during the low-speed rotation (ω: small). However, for instance, when a high load is applied to the servomotor even during the low-speed rotation, a generated torque needs to be increased. Accordingly, the q-axis current Iq (∝ torque) needs to be increased. Then, the drive voltage vector R·Iq is elongated in FIG. 9, so that the end of the total armature voltage vector, i.e., the vector sum, is difficult to stay within the link voltage circle even when the back electromotive force E is small during the low-speed rotation. Consequently, the voltage saturation is easily generated. At this time, the offset voltage vector −ω·L·|Id| needs to be increased to prevent the voltage saturation. However, only a small amount of the d-axis current Id is supplied during the low-speed rotation in the servomotor disclosed in Document 1, so that the offset voltage vector remains small and is not sufficient to prevent the voltage saturation.
As described above, in the servomotor disclosed in Document 1, the amount of the d-axis current Id is variable depending on the rotation angular speed ω. Thus, an excess amount of the d-axis current Id may be supplied during the high-speed rotation, while the voltage saturation may not be appropriately prevented during the low-speed rotation.
On the other hand, in the servomotor disclosed in Document 2, the d-axis current Id and q-axis current Iq are given by the equations of Id=−|I|·sin θ and Iq=I·cos θ, where I denotes a total current flowing through the armature and θ denotes a phase angle (0°≦θ≦90°). The phase angle θ is controlled based on voltage command values serving as command values for armature voltages applied to armature windings of phases.
In the servomotor disclosed in Document 2, the d-axis current Id is defined based on the voltage command values, not the rotation angular speed ω. Thus, the voltage saturation can be prevented without supplying an excess amount of the d-axis current Id even during the high-speed rotation.
However, in the servomotor disclosed in Document 2, the d-axis current Id is supplied every time the voltage command values exceed a voltage command value threshold Vo. Since the voltage saturation is not necessarily generated when the voltage command values exceed the voltage command value threshold Vo, an excess amount of the d-axis current may be also supplied in the servomotor disclosed in Document 2.